fix license notices in source files and license files under GPLv2+

Previously a lot of files were referenced to GNU Public License 2.1,
which never existed.
All copyright attributions are kept the same.

.project

syntax: regexp
^tests/.*/build$
^.idea/.*
syntax: regexp
^.*\.pyc$
syntax: regexp
^.*~$
syntax: regexp
^.*\.swp$

bug_report.*\.txt
i18n/.*.new$
revision